Molded ferrite sheet, sintered ferrite substrate and antenna module

ABSTRACT

The present invention relates to a molded ferrite sheet having opposing surfaces and a thickness in a range of 30 μm to 430 μm, at least one surface of said opposing surfaces having the following surface roughness characteristics (a) to (c):
         (a) a center line average roughness is in a range of 170 nm to 800 nm,   (b) a maximum height is in a range of 3 μm to 10 μm, and   (c) an area occupancy rate of cross-sectional area taken along a horizontal plane at a depth of 50% of the maximum height in a square of side 100 μm is in a range of 10 to 80%.

This application is a divisional of application Ser. No. 12/073,565filed Mar. 6, 2008, allowed, which in turn claims priority to JP2007-057892 filed Mar. 7, 2007 and JP 2007-282576 filed Oct. 31, 2007,the entire contents of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a molded ferrite sheet for use inpreparing a thin single-layer soft magnetic sintered ferrite substrate,to a thin single-layer soft magnetic sintered ferrite substrate, and toan antenna module for a noncontact IC tag using RFID (Radio FrequencyIdentification) technology.

In the fabrication of sintered ferrite substrates, a plurality ofsuperposed molded sheets of a ferrite powder or a mixture of a ferritepowder and a resin are sintered at a time. In this case, the resultingsintered ferrite substrates tend to stick or bond to each other and/orto the sintering table on which they are supported during sintering.When the stuck sintered ferrite substrates are peeled from each other orremoved from the sintering table, the sintered ferrite substrates may bedamaged. In order to prevent the sticking, a releasing powder such aszirconia powder or alumina powder is usually applied to the surfaces ofthe molded ferrite sheets and the sintering table prior to sintering andremoved after sintering. This procedure is very troublesome. Inaddition, the releasing powder is difficult to remove completely fromthe sintered ferrite substrates and thus may contaminate the electronicprecision components in which the sintered ferrite substrates are used.

For example, In Japanese Patent Application Laid-open (KOKAI) No.2-305416 (1990) which relates to a ferrite sheet for supporting aferrite core of a molded ferrite body and preventing deformation thereofduring sintering, it is described, in the description of the relatedart, that alumina powder is customarily provided on a setter asunderlying powder to prevent the deformation by shrinkage duringsintering. The method in which the ferrite sheet is used to preventdeformation of a ferrite core is low in productivity. In addition, theuse of such an additional ferrite sheet is not desirable from thestandpoint of cost. The method using an underlying powder, on the otherhand, has a problem because, especially when the molded ferrite sheetsto be sintered are thin, the obtained sintered ferrite substrates areapt to undulate or break due to physical contact of the substrates withaggregates of the powder formed thereunder during sintering or someother reasons.

In Japanese Patent Application Laid-open (KOKAI) No. 2006-174223, thereis disclosed a method for preparing an antenna-integrated magnetic sheetin which a plurality of 2 mm square ferrite pieces are arranged on asheet substrate and fixedly bonded thereto. Another sheet substrate andan antenna pattern are then placed over the ferrite pieces. Thisapproach is, however, impractical since it is difficult to arrange theferrite pieces regularly on a sheet substrate in an efficient manner.

When conventional antenna modules used in noncontact IC tags using theRFID technology and so on cannot transmit or receive radio waves whenplaced in the vicinity of a metal component because a magnetic flux isconverted into an eddy current by the metal component. As acountermeasure against this drawback, a method is widely used in whichan antenna module having a conductive loop coil is formed in a spiralshape in a plane and a soft magnetic sheet is laminated in parallel tothe coil. In recent years, the demand for size reduction of electronicdevices such as cellular phones and for high-density mounting ofelectronic components is increasing. In addition, the need for anantenna module which is thinner and can provide stable communicationeven if placed in the vicinity of a metal component is increasing moreand more.

In Japanese Patent No. 3,728,320, there is described an invention of anantenna module having a loop coil and a magnetic sheet laminate. Acapacitor is connected in parallel to the loop coil to tune the antennamodule to a desired frequency such as 13.56 MHz prior to installation.However, when the antenna module is incorporated in an electronic deviceand the electronic device is placed in the vicinity of a metalcomponent, the resonant frequency of the antenna may be changed. This isa major problem in practical applications. In Japanese PatentApplication Laid-open (KOKAI) No. 2005-340759, there is described anantenna module having a metallic shield plate. The antenna modulecomprises a ferrite sheet-laminated magnetic member having a thicknessof about 0.5 mm and coated with PET or PPS, and a metallic shield plateattached to the non-communication face of the magnetic member. However,it is difficult to reduce the thickness of an antenna module of thistype, and the recent demand for size reduction of electronic devicescannot be satisfied.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a molded ferrite sheetwhich permits the preparation of clean and thin sintered ferritesubstrates which are not stuck to each other or to the sintering tableeven without a releasing powder such as zirconia powder or aluminapowder. Another object of the present invention is to provide a cleansintered ferrite substrate free of residual releasing powder which maycontaminate electronic devices.

Some antennas for use in the vicinity of a metal component are providedin advance with a magnetic sheet or the like for tuning it to a desiredfrequency. However, when such an antenna is placed in the vicinity ofthe metal component, the resonant frequency of the antenna is changed.Thus, the antennas must be tuned to a desired frequency after they havebeen incorporated in electronic devices. It is, therefore, a furtherobject of the present invention to eliminate such a laborious procedureand to provide a thin antenna module which has a specific frequency thathas been previously adjusted after mounting a magnetic member and whichdoes not undergo any significant change in its frequency even whenplaced in the vicinity of a metal component.

It is yet a further object of the present invention to eliminate theinstability of the antenna characteristics of an antenna for use in thevicinity of a metal component which has been hitherto caused by thepresence of gaps formed between a magnetic member and a metallic shieldplate.

The above-described technical problems can be solved by the presentinvention as follows.

The present invention provides a molded ferrite sheet having opposingsurfaces and a thickness in a range of 30 μm to 430 μm, at least onesurface of said opposing surfaces having the following surface roughnesscharacteristics (a) to (c):

(a) a center line average roughness is in a range of 170 nm to 800 nm,

(b) a maximum height is in a range of 3 μm to 10 μm, and

(c) an area occupancy rate of cross-sectional area taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm is in a range of 10 to 80% (Invention 1).

The present invention also provides a molded ferrite sheet as recited inInvention 1, wherein said at least one surface is roughened bysandblasting (Invention 2).

The present invention further provides a molded ferrite sheet as recitedin Invention 1, wherein said molded ferrite sheet is prepared by moldingunder pressure using a mold or calender roll having a roughened surfaceso that the roughness of the roughened surface of said mold or calenderroll is transferred to a surface of said molded ferrite sheet in contactwith the roughened surface of said mold or calender roll (Invention 3).

The present invention further provides a molded ferrite sheet as recitedin Invention 1, wherein said molded ferrite sheet is prepared by amethod which comprises applying a coating of a ferrite-dispersed coatingliquid to a surface of a plastic film, and drying the applied coating,and wherein said surface of said plastic film has been roughened bysandblasting so that the roughness of said plastic film is transferredto a surface of the dried coating in contact with the roughened surfaceof said plastic film (Invention 4).

The present invention further provides a molded ferrite sheet as recitedin Invention 1, wherein said molded ferrite sheet is prepared by amethod which comprises applying a coating of a ferrite-dispersed coatingliquid to a support and drying the applied coating, and wherein theferrite has been obtained by adjusting a particle size of a ferritepowder having an average particle diameter of 0.1 to 10 μm so that saidsurface roughness characteristics (a) to (c) are imparted to a surfaceof the dried coating in contact with said support (Invention 5).

The present invention further provides a molded ferrite sheet as recitedin Invention 1, wherein the ferrite is Ni—Zn—Cu-based spinel ferrite orMg—Zn—Cu-based spinel ferrite (Invention 6).

The present invention further provides a sintered ferrite substratehaving opposing surfaces and a thickness in a range of 25 μm to 360 μm,at least one surface of said opposing surfaces having the followingsurface roughness characteristics (a) to (c):

(a) a center line average roughness is in a range of 150 nm to 700 nm,

(b) a maximum height is in a range of 2 μm to 9 μm, and

(c) an area occupancy rate of cross-sectional area taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm is in a range of 10 to 80% (Invention 7).

The present invention further provides a sintered ferrite substrate asrecited in Invention 7, wherein the ferrite is Ni—Zn—Cu-based spinelferrite and wherein said sintered ferrite substrate has a magneticpermeability with a real part μr′ and an imaginary part μr″ of not lessthan 80 and not greater than 20, respectively, at 13.56 MHz (Invention8).

The present invention further provides a sintered ferrite substrate asrecited in Invention 7, wherein the ferrite is Mg—Zn—Cu-based spinelferrite and wherein said sintered ferrite substrate has a magneticpermeability with a real part μr′ and an imaginary part μr″ of not lessthan 80 and not greater than 100, respectively, at 13.56 MHz (Invention9).

The present invention further provides a sintered ferrite substrate asrecited in Invention 7, wherein a conductive layer is provided on one ofthe opposing surfaces of the sintered ferrite substrate (Invention 10).

The present invention further provides a sintered ferrite substrate asrecited in Invention 7, wherein grooves are formed on at least one ofthe opposing surfaces of the sintered ferrite substrate (Invention 11).

The present invention further provides a sintered ferrite substrate asrecited in Invention 7 or 8, wherein an adhesive film is adhered to atleast one of the opposing surfaces of the sintered ferrite substrate andwherein the sintered ferrite substrate is divided into a plurality ofparts (Invention 12).

The present invention further provides an antenna module for use in aradio communication medium and a radio communication medium processingdevice, comprising a magnetic member, a conductive loop antenna providedon one side of said magnetic member, and a conductive layer provided onopposite side of said magnetic member from the conductive loop antenna,said magnetic member being a sintered ferrite substrate according toInvention 7 or 8 (Invention 13).

The present invention further provides an antenna module as recited inInvention 13, wherein said conductive layer has a thickness of notgreater than 50 μm and a surface electric resistance of not greater than3 Ω/square (Invention 14).

The present invention further provides an antenna module as recited inInvention 13, wherein said magnetic member is a Ni—Zn—Cu-based spinelsintered ferrite substrate to which a coating of an acrylic resin-basedor epoxy resin-based conductive paint has been applied to form saidconductive layer (Invention 15).

The present invention further provides an antenna module as recited inInvention 13, wherein said magnetic member is a Mg—Zn—Cu-based spinelsintered ferrite substrate to which a coating of an acrylic resin-basedor epoxy resin-based conductive paint has been applied to form saidconductive layer (Invention 16).

The present invention further provides an antenna module as recited inInvention 13, wherein said magnetic member is a Mg—Zn—Cu-based spinelsintered ferrite substrate and wherein said sintered ferrite substrateand said conductive layer provided thereon are formed by forming a printof a silver paste on a molded ferrite sheet, followed by sintering andintegrating said silver paint print and said molded ferrite sheettogether (Invention 17).

The present invention further provides an antenna module as recited inInvention 13, wherein said magnetic member is a Ni—Zn—Cu-based spinelsintered ferrite substrate and wherein said sintered ferrite substrateand said conductive layer provided thereon are formed by forming a printof a silver paste on a molded ferrite sheet, followed by sintering andintegrating said silver paint print and said molded ferrite sheettogether (Invention 18).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view diagrammatically illustrating astructure of an antenna module.

FIG. 2 is an image of the surface shape of a molded ferrite sheet ofExample 2.

FIG. 3 shows an image and the data of area occupancy rate of the moldedferrite sheet of Example 2 obtained from bearing analysis. In thedrawing, an image of the cross-section of the sample taken horizontallyat 50% of the maximum height, a histogram of the heights of surfaceirregularities of the sample, and a graph of their area occupancy ratesare shown.

FIG. 4 is an image of the surface shape of a sintered ferrite substrateof Example 2.

FIG. 5 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Example 2 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 6 is an image of the surface shape of a molded ferrite sheet ofComparative Example 2.

FIG. 7 shows an image and the data of area occupancy rate of the moldedferrite sheet of Comparative Example 2 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 8 is an image of the surface shape of a sintered ferrite substrateof Comparative Example 2.

FIG. 9 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Comparative Example 2 obtained frombearing analysis. In the drawing, an image of the cross-section of thesample taken horizontally at 50% of the maximum height, a histogram ofthe heights of surface irregularities of the sample, and a graph oftheir area occupancy rates are shown.

FIG. 10 is an image of the surface shape of a molded ferrite sheet ofComparative Example 5.

FIG. 11 shows an image and the data of area occupancy rate of the moldedferrite sheet of Comparative Example 5 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 12 is an image of the surface shape of a sintered ferrite substrateof Comparative Example 5.

FIG. 13 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Comparative Example 5 obtained frombearing analysis. In the drawing, an image of the cross-section of thesample taken horizontally at 50% of the maximum height, a histogram ofthe heights of surface irregularities of the sample, and a graph oftheir area occupancy rates are shown.

FIG. 14 is an image of the surface shape of a molded ferrite sheet ofExample 11.

FIG. 15 shows an image and the data of area occupancy rate of the moldedferrite sheet of Example 11 obtained from bearing analysis. In thedrawing, an image of the cross-section of the sample taken horizontallyat 50% of the maximum height, a histogram of the heights of surfaceirregularities of the sample, and a graph of their area occupancy ratesare shown.

FIG. 16 is an image of the surface shape of a sintered ferrite substrateof Example 11.

FIG. 17 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Example 11 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 18 is an image of the surface shape of a molded ferrite sheet ofComparative Example 10.

FIG. 19 shows an image and the data of area occupancy rate of the moldedferrite sheet of Comparative Example 10 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 20 is an image of the surface shape of a sintered ferrite substrateof Comparative Example 10.

FIG. 21 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Comparative Example 10 obtained frombearing analysis. In the drawing, an image of the cross-section of thesample taken horizontally at 50% of the maximum height, a histogram ofthe heights of surface irregularities of the sample, and a graph oftheir area occupancy rates are shown.

FIG. 22 is an image of the surface shape of a molded ferrite sheet ofComparative Example 13.

FIG. 23 shows an image and the data of area occupancy rate of the moldedferrite sheet of Comparative Example 13 obtained from bearing analysis.In the drawing, an image of the cross-section of the sample takenhorizontally at 50% of the maximum height, a histogram of the heights ofsurface irregularities of the sample, and a graph of their areaoccupancy rates are shown.

FIG. 24 is an image of the surface shape of a sintered ferrite substrateof Comparative Example 13.

FIG. 25 shows an image and the data of area occupancy rate of thesintered ferrite substrate of Comparative Example 13 obtained frombearing analysis. In the drawing, an image of the cross-section of thesample taken horizontally at 50% of the maximum height, a histogram ofthe heights of surface irregularities of the sample, and a graph oftheir area occupancy rates are shown.

DESCRIPTION OF REFERENCE NUMERALS

-   1: sintered ferrite substrate-   2: insulator film-   3: conductive layer-   4: double coated adhesive tape-   5: conductive loop-   6: separator member

DETAILED DESCRIPTION OF THE INVENTION

A molded ferrite sheet according to the present invention will be firstdescribed.

The molded ferrite sheet of the present invention has at least onesurface having the following surface roughness characteristics (a) to(c):

(a) a center line average roughness (Ra) is in a range of 170 nm to 800nm,

(b) a maximum height (Rmax) is in a range of 3 μm to 10 μm, and

(c) an area occupancy rate of cross-sectional area taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm is in a range of 10 to 80%.

Preferably, the center line average roughness is in a range of 180 to700 nm, and the maximum height is in a range of 4 to 8 μm. In thepresent invention, it is necessary to control the density of surfaceirregularities, which cannot be expressed by the center line averageroughness and the maximum height. In a bearing analysis performed on a100 μm box image to measure the surface roughness of the molded ferritesheet, the area occupancy rate of cross-sectional area taken along ahorizontal plane (in parallel with the sheet) at a depth of 50% of themaximum height is in a range of 10 to 80%, preferably in a range of 15to 75%, based on the box area (10,000 μm²). When the area occupancy rateis within this range, the molded ferrite sheets are not stuck or bondedto each other during sintering even when the sheets are superposed oneupon another and no releasing powder is used. As a result, sinteredferrite substrates intended by the present invention can be obtained.

When the center line average roughness is less than 170 nm or themaximum height is less than 3 μm, the sheets are stuck to each otherduring sintering. When the center line average roughness is greater than800 nm or the maximum height is greater than 10 μm, the molded sheet hasso large a contact area that the molded sheet is difficult to releasefrom molds. In addition, since the resulting sintered ferrite substratelacks surface smoothness, it breaks easily. Further, gaps tend to beformed between the molded ferrite sheet and an insulator film orconductive layer provided thereon. Furthermore, the resulting sinteredferrite substrate has only a small sintered cross-sectional area andthus has a low magnetic permeability, resulting in poor antennacharacteristics. The surface roughness is critical especially when thesintered ferrite substrate has a small thickness of not greater than 200μm. When the area occupancy rate of cross-sectional area taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm is less than 10% or greater than 80%, superposed sheets arestuck to each other during sintering and the obtained sintered ferritesubstrates will be difficult to separate from each other.

The ferrite powder for the molded ferrite sheet of the present inventionis preferably an Ni—Zn—Cu spinel ferrite powder or an Mg—Zn—Cu spinelferrite powder. When an Ni—Zn—Cu spinel ferrite powder is used, it ispreferably composed of 40 to 50 mol % of Fe₂O₃, 10 to 30 mol % of NiO,10 to 30 mol % of ZnO and 0 to 20 mol % of CuO. When an Mg—Zn—Cu spinelferrite powder is used, it is preferably composed of 40 to 50 mol % ofFe₂O₃, 15 to 35 mol % of MgO, 5 to 25 mol % of ZnO and 0 to 20 mol %CuO. The ferrite powder can be obtained by uniformly mixing oxide powderingredients, calcining the mixture at 750° C. to 950° C. for two hours,and pulverizing the calcined product. The use of a ferrite powder havinga cumulative 50% volume diameter of 0.5 to 1.0 μm is preferred.

A method for producing the molded ferrite sheet according to the presentinvention will be next described.

Although the method for obtaining the molded ferrite sheet of thepresent invention is not specifically limited, a sandblasting method,which is widely used in the field of metal polishing, can be used forroughening a surface of the molded ferrite sheet of the presentinvention. That is, the molded ferrite sheet with a roughened surfacecan be obtained by injecting an aqueous solution in which a polishingmaterial such as glass or alumina is dispersed onto a molded ferritesheet and then washing the molded ferrite sheet with water.

In another method for obtaining the molded ferrite sheet of the presentinvention, a melt of a mixture of a ferrite powder and a thermoplasticplastic is used. The melt is molded into a sheet under pressure using acalender roll or mold having a treated (roughened) surface. Examples ofusable thermoplastic resins include polyethylene (PE), polypropylene(PP) and polyvinyl butyral (PVB). A thermoplastic plastic elastomer suchas styrene-ethylene-butylenes-based resins and olefin-based resins canbe also used. When necessary, two or more thermoplastic resins and/orthermoplastic plastic elastomer may be used in combination. A mixture of1000 parts by weight of a coupling agent-treated ferrite powder,obtained by treating 1000 parts by weight of a ferrite powder with 10 to50 parts by weight of a coupling agent, and 70 to 120 parts by weight ofa resin is kneaded in a pressure kneader or the like kneading device at120 to 140° C. for 20 to 60 minutes and the kneaded mixture is formed bymolding under pressure using mold having a roughened surface. It ispreferred to use a low-density polyethylene (LDPE) or polyvinyl butyral(PVB) since they are decomposed during sintering. The thermoplasticresin is preferably used in an amount of 70 to 110 parts by weight per1000 parts by weight of the coupling agent-treated ferrite.

A further method for obtaining the molded ferrite sheet of the presentinvention is to apply a coating of a ferrite-dispersed coating liquid toa surface of a plastic film. The ferrite-dispersed coating liquid ispreferably composed of 1000 parts by weight of an Ni—Zn—Cu ferritepowder, 70 to 120 parts by weight of a polyvinyl alcohol resin, 15 to 25parts by weight of butyl butylphthalate as a plasticizer and 400 to 600parts by weight of a solvent. Examples of usable solvents include glycolether solvents, MEK, toluene, methanol, ethanol and n-butanol. In viewof the dispersiblity of the ferrite powder and for ease of mixing anddrying, the coating liquid is preferably composed of 1000 parts byweight of ferrite, 80 to 110 parts by weight of a polybutyral resin, 18to 22 parts by weight of butyl butylphthalate and 450 to 550 parts byweight of a solvent.

The method for preparing the coating liquid is not specifically limitedbut the use of a ball mill is preferred. When the solvent and ferriteare first mixed in a ball mill and then the mixture is mixed with theresin and plasticizer, a uniform coating liquid can be obtained. It isimportant that the thus obtained coating liquid should be subjected tovacuum defoaming in a vacuum vessel sufficiently to prevent cracking ofthe coated film during drying.

The method for coating the ferrite-dispersed coating liquid is notspecifically limited. A roll coater or doctor blade can be used. The useof a doctor blade is preferred since it can form a layer with highthickness accuracy and does not affect the stability of the coatingliquid. The coating liquid is applied to a plastic film with a doctorblade to form a layer of a desired thickness. The applied layer is thendried at 80 to 130° C. for 30 to 60 minutes to obtain a molded ferritesheet.

The plastic film on which the ferrite-dispersed coating liquid is to beapplied is not specifically limited. A sandblasted polyethylene (PE),polypropylene (PP), polyethylene terephthalate (PET) or polyimide filmcan be suitably used. The use of a polyethylene terephthalate (PET) filmis preferred for ease of surface treatment and thermal stability duringthe coating and drying processes. When a sandblasted plastic film isused, the surface roughness (surface irregularities) of the plastic filmcan be transferred to the molded ferrite sheet, whereby a molded sheethaving a desired surface roughness can be obtained.

Yet a further method for obtaining the molded ferrite sheet of thepresent invention is to control the surface roughness of the moldedferrite sheet by adjusting the particle diameter of the ferrite powder.When a mixture of 100 parts by weight of a ferrite powder having acumulative 50% volume diameter of 0.1 to 1.0 μm and 5 to 40 parts byweight of a ferrite powder having a cumulative 50% volume diameter of 3to 10 μm is used to form a ferrite-dispersed coating liquid, a moldedferrite sheet having a surface roughness as required in the presentinvention can be obtained by applying the coating liquid to a surface ofa support. The support may be a sandblasted plastic film. However, therequired surface roughness may be obtained even when the support has notbeen sandblasted. In view of the surface roughness of the sheet, it ispreferred to mix 100 parts by weight of a ferrite powder having acumulative 50% volume diameter of 0.3 to 0.7 μm with 10 to 40 parts byweight of a ferrite powder having a cumulative 50% volume diameter of 3to 7 μm.

In the present invention, the molded ferrite sheet is heat-treated toobtain a sintered ferrite substrate.

In the heat treatment, 5 to 20 molded ferrite sheets of the presentinvention are generally stacked on an alumina support plate having aporosity of 30% and sintered at a time in an electric furnace or thelike. It is important to control the heat treatment conditions so thatremoval of the resin component and growth of the ferrite particlesproceed effectively. To remove the resin component, the molded ferritesheets are preferably maintained at 150° C. to 550° C. for 5 to 80hours. To grow the ferrite particles, the molded ferrite sheets are thenpreferably maintained at 850° C. to 1200° C. for 1 to 5 hours.

To prevent thermal deformation and/or cracking of the sheets, it ispreferred that the temperature be raised from room temperature at a rateof 10 to 20° C./hour and then maintained constant in the process ofremoving the resin component. It is also preferred that the temperaturebe then raised at a rate of 30 to 60° C./hour, maintained constant tosinter the molded ferrite sheets until the ferrite particles growssufficiently. The sintered product is then gradually cooled. Theretention time and temperature in each process are suitably selecteddepending on the number of the molded ferrite sheets to be treated.

The sintered ferrite substrate according to the present invention willbe next described.

The sintered ferrite substrate of the present invention has a surfaceroughness such that the center line average roughness (Ra) is in a rangeof 50 to 700 nm and the maximum height (Rmax) is in a range of 2 to 9μm. Preferably, the center line average roughness (Ra) is in a range of160 to 600 nm and the maximum height is in a range of 3 to 8 μm.

In addition, the area occupancy rate of cross-sectional area taken alonga horizontal plane at a depth of 50% of the maximum height in a squareof side 100 μm is in a range of 5 to 70%, preferably in a range of 10 to60%, more preferably in a range of 10 to 50%.

When the molded ferrite sheet as described before is sintered, asintered ferrite substrate having a center line average roughness (Ra)of not less than 150 nm and a maximum height (Rmax) of not less than 2μm can be obtained. When the center line average roughness (Ra) isgreater than 700 nm or the maximum height (Rmax) is greater than 9 μm,the sintered ferrite substrate lacks surface smoothness and breakseasily. Also, gaps tend to be formed between the sintered ferritesubstrate and an insulator film or a conductive layer provided thereon,resulting in poor antenna characteristics. Further, the resultingsintered ferrite substrate has only a small sintered cross-sectionalarea and thus has a low magnetic permeability. The surface roughness iscritical especially when the sintered ferrite substrate has a smallthickness of 200 μm or less.

When the molded ferrite sheet as described before is sintered, the areaoccupancy rate of cross-sectional area taken along a horizontal plane ata depth of 50% of the maximum height in a square of side 100 μm will bein a range of 5 to 70%.

The sintered ferrite substrate according to the present inventionpreferably has a sintered density in a range of 4.6 to 5.0 g/cm³. Whenthe sintered ferrite substrate has a sintered density below 4.4 g/cm³,it is not sufficiently sintered. In this case, the sintered ferritesubstrate breaks easily and has a magnetic permeability with a low realpart μr•. However, it is not necessary to sinter the molded ferritesheet until the sintered density exceeds 5.0 g/cm³. The sintered densityis more preferably in a range of 4.5 to 4.9 g/cm³.

Bendability may be suitably imparted to the sintered ferrite substrateof the present invention by dividing the substrate into parts with anadhesive film provided on at least one side thereof. When the sinteredferrite substrate is divided into parts, its magnetic permeability islowered. However, the magnetic permeability changes depending on how itis divided. Thus, when grooves are formed in a surface of the substrateat regular intervals so that the sintered ferrite substrate can beeasily divided along the grooves, the magnetic characteristics of thesintered ferrite substrate divided into parts and provided withbendability can be stabilized.

To provide the sintered ferrite substrate of the present invention withgrooves, V-shaped grooves with an apex angle of 25 to 45 degrees may beformed in one side of the molded sheet with an embossing roll or metalblade. The grooves are formed such that the distance between the bottomsof the adjacent two grooves is in a range of 1 to 5 mm. When thedistance is less than 1 mm, the magnetic permeability is lowered whenthe sintered ferrite substrate is divided along the grooves. Further,such narrow spaced grooves are difficult to form. On the other hand,when the distance is greater than 5 mm, the sintered ferrite substratecannot have sufficient flexibility. The distance between the grooves ispreferably in a range of 2 to 4 mm.

The depth of the grooves is in a range of 0.4 to 0.7 as expressed interms of the ratio of the depth to the thickness of the molded sheet(groove depth/sheet thickness ratio). When the groove depth/sheetthickness ratio is less than 0.4, the sintered ferrite substrate may notbe divided along the grooves but be broken irregularly, resulting inunstable magnetic permeability. When the groove depth is greater than0.7, the molded ferrite sheet may be broken along the grooves duringsintering. Preferably, the groove depth/sheet thickness ratio is in arange of 0.4 to 0.6.

The pattern of the grooves to be formed in a sheet surface may be anysuitable pattern such as equilateral triangles, grids or polygons. It isimportant that when the sintered ferrite substrate is divided into partsalong the grooves, the parts should be as uniform as possible in sizeand shape and that the magnetic permeability of the substrate shouldhardly change even when the resulting substrate is bent.

The sintered ferrite substrate of the present invention is thin andbreaks easily. However, it has been found that when the sintered ferritesubstrate produced from an Ni—Zn—Cu spinel ferrite powder is dividedinto parts with an adhesive protective film provided on at least oneside thereof, and when the real part μr′ and the imaginary part μr″ ofthe magnetic permeability at 13.56 MHz are maintained at not less than80 and not greater than 20, respectively, the sintered ferrite substratehas adequate flexibility and is very excellent as a thin sinteredferrite substrate with a thickness in a range of 25 to 360 μm for a loopantenna module.

When the real part μr′ of the magnetic permeability of the sinteredferrite substrate made from an Ni—Zn—Cu spinel ferrite powder is lessthan 80, the resulting antenna module has a low coil inductance and cantransmit and receive radio waves over only a short distance. When theimaginary part μr″ of the magnetic permeability is greater than 20, losswill increase and the antenna has a low resonance Q and can transmit andreceive radio waves over only a short distance. The imaginary part μr″is preferably not greater than 10, more preferably not greater than 5.

It has been found that when a sintered ferrite substrate is producedfrom an Mg—Zn—Cu spinel ferrite powder, and when the real part μr′ andimaginary part μr″ of the magnetic permeability at 13.56 MHz aremaintained at not less than 80 and not greater than 100, respectively,the sintered ferrite substrate has suitable flexibility and is veryexcellent as a thin sintered ferrite substrate with a thickness of 25 to360 μm for a loop antenna module.

When the real part μr′ of the magnetic permeability of the sinteredferrite substrate made from an Mg—Zn—Cu spinel ferrite powder is lessthan 80, the resulting antenna module has a low coil inductance and cantransmit and receive radio waves over only a short distance. When theimaginary part μr″ of the magnetic permeability is greater than 150,loss will increase and the antenna has a low resonance Q and cantransmit and receive radio waves over only a short distance. The realpart μr′ and the imaginary part μr″ of the magnetic permeability arepreferably not less than 85 and not greater than 90, respectively.

An antenna module according to the present invention will be nextdescribed.

The antenna module of the present invention comprises a sintered ferritesubstrate as a magnetic member, a conductive loop antenna provided onone side of the sintered ferrite substrate, and a conductive layerprovided on opposite side of the sintered ferrite substrate from theantenna. The conductive loop antenna comprises an insulator film, suchas polyimide film or PET film, with a thickness in a range of 20 to 60μm and a spiral conductive loop with a thickness in a range of 20 to 30μm provided on one side of the insulator film. The conductive layer maybe formed on a sintered ferrite substrate with a thickness in a range of25 to 360 μm by coating a conductive paint on one side thereof anddrying the coated paint. Alternatively, the conductively layer may beformed by forming a print of a silver paste on a molded ferrite sheetand sintering and integrating the silver paste print and the moldedferrite sheet together.

The conductive layer preferably has a thickness in a range of 5 to 50μm. When the conductive loop antenna is attached to opposite side of thesintered ferrite substrate from the conductive layer with a doublecoated adhesive tape having a thickness in a range of 20 to 60 μm andwhen the same adhesive tape is provided on the conductive layer, anantenna module with a total thickness in a range of 110 to 620 μm asshown in FIG. 1 can be obtained.

The insulator film is not specifically limited but preferably has asurface electric resistance of not less than 5 MΩ/square, preferably notless than 10 MΩ/square, to prevent minute leakage current.

The conductive paint may be prepared by dispersing copper or silverpowder as a conductive filler in a mixture of an acrylic resin or epoxyresin and an organic solvent such as butyl acetate or toluene.

The conductive paint is applied to one side of the sintered ferritesubstrate and dried and cured in atmosphere at a temperature of roomtemperature to 100° C. for 30 minutes to 3 hours to form a conductivelayer with a thickness in a range of 20 to 50 μm. The conductive layerpreferably has a surface electric resistance of not greater than 3Ω/square. To reduce changes in antenna characteristics when the antennamodule is placed in the vicinity of a metal component, the surfaceelectric resistance is desired to be not greater than 1 Ω/square. Thethickness of the conductive layer may be in the range of 20 to 30 μm toreduce the total thickness of the antenna module.

A sintered ferrite substrate with a conductive layer can be obtained bysintering and integrating a molded ferrite sheet and a conductive pastelayer provided on one side of the molded ferrite sheet by a green sheetmethod. To prevent the conductive layer from exposing in an electronicdevice, an insulating protective film may be provided thereon. Acapacitor is connected in parallel to the loop to adjust and tune thethus obtained antenna module to a resonant frequency of to 13.56 MHz asis well known in the art so that the antenna module can resonate at adesired frequency.

The antenna module having a conductive loop antenna, an adhesive layer,a sintered ferrite substrate and a conductive layer which are closelybonded and integrated together and a capacitor connected in parallel tothe loop circuit to tune the resonant frequency of the antenna module to13.56 MHz as described above undergoes no significant change in itsantenna characteristics and can provide stable radio communication evenwhen placed in the vicinity of a metal component of an electronic deviceof various types.

According to the present invention, clean and thin molded ferrite sheetswhich are not stuck or bonded without any previous releasing treatmentusing a zirconia powder or alumina powder, and sintered ferritesubstrates free of releasing powder which may contaminate electronicdevices in which they are used can be provided.

According to the preferred embodiment of the present invention, thesintered ferrite substrate having a thickness in a range of 25 to 360 μmand a magnetic permeability with a real part μr′ and an imaginary partμr″ of not less than 80 and not greater than 20, respectively (when theferrite is an Ni—Zn—Cu spinel ferrite) or not less than 80 and notgreater than 100, respectively (when the ferrite is an Mg—Zn—Cu spinelferrite), at 13.56 MHz can be suitably used as a magnetic member for anantenna module. The sintered ferrite substrate can contributesignificantly to the reduction in thickness of an antenna module.

According to the preferred embodiment the of present invention, since athin conductive layer is formed by a coating or printing method on asintered ferrite substrate as a magnetic member of an antenna module tobe used in the vicinity of a metal component, the total thickness of theantenna module can be as small as 100 to 580 μm. Also, according to thepresent invention, since the resonant frequency is adjusted after theconductive loop coil, magnetic member (soft magnetic layer) andconductive layer have been integrated together, the antenna undergoeslittle change in its characteristics after being incorporated in adevice. Thus, it requires no complicated adjustment after beingincorporated in a device.

According to the preferred embodiment of the present invention, since nogap can be formed between a magnetic member and a conductive layer in anantenna module to be used in the vicinity of a metal component, theantenna module has very stable antenna characteristics.

According to the preferred embodiment of the present invention, sincethe sintered ferrite substrate as a magnetic member is provided with anadhesive film on at least one side thereof and is divided into parts toprovide it with bendability, the antenna module is easy to handle andchanges in characteristics of the antenna module caused by breaking ofthe magnetic member can be minimized.

According to the present invention, since a desired roughness can beeasily imparted to at least a surface of the molded ferrite sheet,molded ferrite sheets which are not bonded even if sintered in a stackwithout using a releasing powder can be produced industrially.

The sintered ferrite substrate according to the present invention has arelatively small thickness but has a high magnetic permeability. Inaddition, since a plurality of such sintered ferrite substrates can besintered in a stack without using a releasing powder such as zirconia oralumina powder, the sintered ferrite substrate does not causecontamination by such a powder when being incorporated in an electronicdevice. Therefore, the sintered ferrite substrate is highly suitable asa magnetic core of a high-density mounted antenna module for RFID (RadioFrequency Identification) tags, which are widely used in recent years.

An antenna module obtained by integrally combining a sintered ferritesubstrate of the present invention provided with a conductive layer witha thickness in a range of 20 to 50 μm on one side thereof and aconductive loop antenna does not undergo any significant change incharacteristics even when incorporated in an electronic device for RFID(Radio Frequency Identification) communication and placed in thevicinity of a metal component and has a relatively small thickness.Therefore, the antenna module can meet the demand for high-densitymounting of electronic devices.

EXAMPLES

The measurement methods used in the following examples will bedescribed.

Surface Roughness:

The surface roughnesses of the molded ferrite sheet and sintered ferritesubstrate were determined by measuring the center line average roughnessRa and maximum height Rmax in a region of a square of side 100 μm usingan atomic force microscope AFM (Nano Scope III, manufactured by DigitalInstrument).

To determine the surface irregularity in terms of height distribution,bearing analysis software of the apparatus was used. When the areaoccupancy rate of cross-sectional area taken along a horizontal plane ata depth of 50% of the maximum height (Rmax) in a square of side 100 μmis obtained on the image used to obtain the surface roughness, thesurface profile can be compared with that of other samples. When thearea occupancy rate is in a range of 10 to 80%, sticking or bonding ofthe molded ferrite sheets during sintering can be prevented. When thearea occupancy rates of sintered ferrite substrates after sintering weremeasured, sintered ferrite substrates which were not stuck to each otherhad an area occupancy rate in a range of 5 to 70%. The surface roughnessof a sintered ferrite substrate of Comparative Examples which was stuckheavily was measured at portion on a broken piece where no stickingoccurred.

Cumulative 50% Volume Diameter:

The average particle diameter of the ferrite powder was measured by awet method using Microtrack MT3300 manufactured by Nikkiso Co. Ltd. 5Gram of the ferrite powder was added to 100 ml of an aqueous solutioncontaining 0.2% of hexametaphosphoric acid as a dispersant and 0.05% ofa nonionic surfactant (Triton X-100 manufactured by Dow ChemicalCompany) as a surfactant and the resulting mixture was dispersed for 300seconds using an ultrasonic homogenizer (Type 300W manufactured byNikkiso Co. Ltd.). Then, the volumetric distribution was measured underthe following conditions; measuring time: 30 seconds, measurement range:from 0.021 to 1408 μm, solvent refractive index: 1.33, particlerefractive index: 2.94 and particle shape: nonspherical.

Sheet Thickness:

The thicknesses of the four corners of a sample piece with an externalsize of 80 mm×80 mm cut from the molded sheet were measured withDigimatic Indicator ID-S112 manufactured by Mitsutoyo Corporation andthe average of the thicknesses was taken as the thickness of the sheet.

Sintered Density:

The sintered density of the sintered ferrite substrate was calculatedfrom its volume obtained from outer dimensions thereof and its weight.

Magnetic Permeability:

The sintered ferrite substrate was cut into a test piece having a ringshape with an outer diameter of 14 mm and an inner diameter of 8 mm andits thickness was measured. The magnetic permeability of the test pieceat a frequency of 13.56 MHz was measured using an Impedance AnalyzerHP4291A (manufactured by Hewlett-Packard Development Company) and a jig(HP1645A) attached to the test station thereof or Impedance Analyzer(E4991A manufactured by Agilent Technologies Co., Ltd.) and a jig(16454A) attached to its test station.

Resonant Frequency and Degree of Resonance:

The resonance characteristics of the antenna module were obtained bymeasuring the resonance characteristics of a conductive loop antennahaving a structure as shown in FIG. 1. The frequency characteristics ofthe impedance of the power supply line for the laminated antenna moduleconfigured as shown in FIG. 1 were measured. A capacitor was connectedin parallel to the power supply line and its capacitance was adjusted.Then, when the resonant frequency became 13.56 MHz, the degree ofresonance Q was measured using an impedance analyzer HP4291Amanufactured by Hewlett-Packard Development Company. In Examples 7 to 9and 16 to 18, and Comparative Examples 6 to 8 and 14 to 16, the resonantfrequency and the degree of resonance Q were measured under the sameconditions as those set for the case where no iron plate was laminatedfor comparison.

Surface Electric Resistance:

The surface electric resistance of the conductive layer was measured bya four probe method (according to JISK 7149) using an electricresistivity meter Loresta-GP (MCP-T600 manufactured by MitsubishiChemical Corporation).

Example 1

Using a pressure kneader, 1,000 parts by weight of a ferrite powder,obtained by surface-treating 1,000 parts by weight of Ni—Zn—Cu ferritepowder (composition: Fe₂O₃: 48.5 mol %, NiO: 20.5 mol %, ZnO: 20.5 mol%, CuO: 10.5 mol %; calcinations conditions: 850° C., 90 minutes;cumulative 50% volume diameter: adjusted to 0.7 μm) with 10 parts byweight of a titanate-based coupling agent (KR-TTS manufactured byAjinomoto Co., Inc.), 50 parts by weight of a thermoplastic elastomer(LUMITAC 22-1 manufactured by Tosoh Corporation), 100 parts by weight ofpolyethylene having a density of 0.9 g/cm³ and 20 parts by weight ofstearic acid were kneaded at 130° C. for 40 minutes. The thus obtainedkneaded mass of a ferrite resin composition was press molded at atemperature of 160° C. under a pressure of 100 kg/cm² for apressurization time of 3 minutes using an iron plate, which had beensandblasted to have a center line average roughness of 450 nm and amaximum height of 8 μm, to obtain a molded ferrite sheet having athickness of 77 μm and a size of 100 mm square. Ten such sheets wereprepared and stacked one upon another. The stacked substrates wereplaced between top and bottom alumina setters (manufactured by KikusuiChemical Industries Co., Ltd.) as support plate means and heated at 500°C. for 10 hours for removing the organic binder, followed by sinteringat 920° C. for 2 hours to obtain sintered substrates. After cooling, thesintered substrates were peeled from each other. It was found that thesintered substrate was able to be easily peeled off without causing anydamage. The sintered substrate had a thickness of 65 μm and an outerdimension of 80 mm square. A test piece having an outer diameter of 14mm and an inner diameter of 8 mm was cut out from the substrate andmeasured for its magnetic permeability using Impedance Analyzer (HP4291Amanufactured by Hewlett-Packard Inc.) and a jig (HP16454A) attached toits test station. It was found that μr′ and μr″ were 98 and 2.2,respectively, at 13.56 MHz. Thus, the obtained sintered ferritesubstrates were not stuck to each other and had good magneticcharacteristics.

The above-obtained molded ferrite sheet was found to have a surfaceroughness such that the center line average roughness was 420 nm, themaximum height was 6.5 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 48%. The obtainedsintered ferrite substrate was found to have a surface roughness suchthat the center line average roughness was 400 nm, the maximum heightwas 5.5 μm and an area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 45%.

Example 2

Using a ball mill, 100 parts by weight of the same Ni—Zn—Cu ferrite asused in Example 1, 2 parts by weight of butyl phthalyl butyl glycolate,12 parts of a polyvinylalcohol resin (ESLEK B BM-1 manufactured bySekisui Chemical Co., Ltd.) and 60 parts by weight of a mixed solventcomposed of 4 parts of n-butanol and 6 parts of toluene were mixed,dissolved or dispersed to obtain a ferrite-dispersed coating liquid. Theferrite-dispersed coating liquid was defoamed by an oil rotary vacuumpump and uniformly applied with a doctor blade to a PET film (LUMIMAT50S200 TRES manufactured by Panak Co., Ltd.), one side of which had beensandblasted to have a center line average roughness of 530 nm and amaximum height of 5.6 μm, to a given thickness. The coating was driedwith hot wind at 100° C. for 30 minutes to obtain a molded ferrite sheethaving a thickness of 204 μm. The molded ferrite sheet was cut intosquares of side 100 mm. Each of the cut sheets was peeled off from thePET film. The sheets were then sintered in the same conditions as thosein Example 1 to obtain sintered ferrite substrates. The obtainedsintered ferrite substrate was evaluated for its physical properties andwas found to have a thickness of 160 μm, an outer dimension of 80 mmsquare and magnetic permeability with μr′ of 96 and μr″ of 3. Further,the obtained sintered ferrite substrates did not stick to each other andwere easily peeled off from each other.

The above-obtained molded ferrite sheet was found to have a surfaceroughness such that the center line average roughness was 370 nm, themaximum height was 4.0 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 73%. The surfaceof the above-obtained molded ferrite sheet which had not been broughtinto contact with the PET film was found to have a center line averageroughness of 104 nm, a maximum height of 1.3 μm and an area occupancyrate of the cross-sectional area, taken along a horizontal plane at adepth of 50% of the maximum height in a square of side 100 μm, of 93%.Thus, it was understood that the surface roughness was able to becontrolled at will by the PET film used. The above-obtained moldedferrite sheet was found to have a surface roughness such that the centerline average roughness was 292 nm, the maximum height was 3.5 μm and anarea occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, was 12%.

Example 3

A kneaded mass of a ferrite resin composition was prepared in the samemanner as that in Example 1 except that a mixture of 300 parts by weightof Ni—Zn—Cu ferrite powder (composition: Fe₂O₃: 48.5 mol %, NiO: 20.5mol %, ZnO: 20.5 mol %, CuO: 10.5 mol %; calcination conditions: 1,000°C., 90 minutes; cumulative 50% volume diameter: 6 μm) and 700 parts byweight of the same Ni—Zn—Cu ferrite powder (cumulative 50% volumediameter: 0.7 μm) as used in Example 1 was used. The thus obtainedkneaded mass was press molded at a temperature of 160° C. under apressure of 100 kg/cm² for a pressurization time of 3 minutes using aniron plate, which had been processed to have a center line averageroughness of 120 nm and a maximum roughness of 2 μm, to obtain a moldedferrite sheet having a thickness of 200 μm and an outer size of 100 mm.Using the thus obtained sheet, sintered ferrite substrates were preparedin the same manner as that in Example 1. The obtained sintered ferritesubstrate was evaluated for its physical properties and was found tohave a thickness of 167 μm and magnetic permeability with μr′ of 80 andμr″ of 1.1 at 13.56 MHz. Further, the substrates did not stick to eachother and were easily peeled off from each other.

Because of the use of coarse particle ferrite, the above-obtained moldedferrite sheet had a center line average roughness of 270 nm, a maximumheight of 5.0 μm and an area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, of 20%. The sintered ferrite substrate hadsurface roughness such that the center line average roughness was 250nm, the maximum height was 4.0 μm and the area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 18%.

Example 4

Sintered ferrite substrates were obtained in the same manner as that inExample 2 except that the application of the ferrite-dispersed coatingliquid using the doctor blade was carried out under conditions so that amolded ferrite sheet obtained had a thickness of 42 μm. The obtainedsintered ferrite substrate was evaluated for its physical properties andwas found to have a thickness of 37 μm and magnetic permeability withμr′ of 95 and μr″ of 2 at 13.56 MHz. Further, the substrates did notstick to each other and were easily peeled off from each other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness of 435 nm, the maximum height of 6.3 μmand the area occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, was 52%. The sintered ferrite substrate had surfaceroughness such that the center line average roughness was 425 nm, themaximum height was 4.9 μm and the area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 50%.

Example 5

Sintered ferrite substrates were obtained in the same manner as that inExample 2 except that the application of the ferrite-dispersed coatingliquid using the doctor blade was carried out under conditions so that amolded ferrite sheet obtained had a thickness of 405 μm. The obtainedsintered ferrite substrate was evaluated for its physical properties andwas found to have a thickness of 350 μm and magnetic permeability withμr′ of 102 and μr″ of 3.2 at 13.56 MHz. Further, the substrates did notstick to each other and was easily peeled off from each other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness of 409 nm, the maximum height of 6.8 μmand the area occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, was 58%. The sintered ferrite substrate had surfaceroughness such that the center line average roughness was 388 nm, themaximum height was 5.6 μm and the area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 41%.

Comparative Example 1

A kneaded mass of a ferrite resin composition was prepared in the samemanner as that in Example 1. Using the kneaded mass, sintered ferritesubstrates were prepared in the same manner as that in Example 1 exceptthat an iron plate processed to have a center line average roughness of120 nm and a maximum roughness of 2 μm was used in the press molding.The substrates stuck so tightly to each other that it was difficult topeel off the substrates from each other. Although the substrate waspartly unstuck from each other, breakage occurred when it was forciblyseparated therefrom. Thus, no sintered ferrite substrates having a sizeof 80 mm square were obtained. The obtained sintered ferrite substratehad magnetic permeability with μr′ of 98 and μr″ of 1.9 at 13.56 MHz.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness of 115 nm, the maximum height of 1.8 μmand the area occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, was 2%. The sintered ferrite substrate had surfaceroughness such that the center line average roughness was 98 nm, themaximum height was 1.1 μm and the area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 2%.

Comparative Example 2

A ferrite-dispersed coating liquid was prepared in the same manner asthat in Example 2. The obtained coating liquid was uniformly appliedwith a doctor blade to a PET film (not sandblasted) having a center lineaverage roughness of 17 nm, a maximum height of 0.3 μm and a thicknessof 50 μm. This was dried with hot wind at 100° C. for 30 minutes toobtain a molded ferrite sheet having a thickness of 202 μm. The sheetwas peeled off from the PET film. Ten such sheets were stacked one uponanother and heated in the same manner as that in Example 0.1 to obtainsintered ferrite substrates. The obtained substrate was evaluated forits properties and was found to have a thickness of 165 μm. Since thesubstrates stuck so tightly to each other it was not possible to peeloff the substrates from each other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness of 66 nm, the maximum height of 1.3 μm andthe area occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, was 90%. The sintered ferrite substrate had surfaceroughness such that the center line average roughness was 44 nm, themaximum height was 0.9 μm and the area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 1%.

Comparative Example 3

A sheet was prepared in the same manner as that in Comparative Example 2and was peeled off from the PET film. That surface of the molded ferritesheet which had been in contact with the PET film was applied with 50 mgof zirconia powder having an average particle diameter of 5 μm bybrushing. Thereafter, a calcination treatment was carried out in thesame manner as that in Comparative Example 2 to obtain sintered ferritesubstrates. The evaluation of the sintered ferrite substrate revealedthat the substrate had magnetic permeability with μr′ of 96 and μr″ of1.8 at 13.56 MHz. The obtained sintered ferrite substrates had depositsof zirconia powder. During the removal of the zirconia powder depositsusing a brush, three of the ten substrates were broken. The applicationof the powder and the removal of the powder were significantlytroublesome. The powder deposits were not able to be completely removed.

Comparative Example 4

Sintered ferrite substrates were prepared in the same manner as that inExample 1 except that an iron plate processed to have a center lineaverage roughness of 1200 nm and a maximum height of 14 μm was used inthe press molding. The substrates did not stuck to each other and wereable to be peeled off from each other. The obtained sintered ferritesubstrate had magnetic permeability with μr′ of 75 and μr″ of 0.6 at13.56 MHz and, thus, did not have satisfactory magnetic characteristics.Such deterioration of the magnetic permeability is considered to beattributed to the high surface roughness of the iron plate whichresulted in an increase of void spaces in the cross-sections of thesintered ferrite substrate.

Comparative Example 5

A ferrite-dispersed coating liquid was prepared in the same manner asthat in Example 2. The obtained coating liquid was uniformly appliedwith a doctor blade to a PET film (U4-50 manufactured by Teijin DuPontCo., Ltd) which had been processed to have a center line averageroughness of 252 nm and a maximum height of 3.3 μm. The coating wasdried with hot wind at 100° C. for 30 minutes to obtain a molded ferritesheet having a thickness of 200 μm. The sheet was peeled off from thePET film. Ten such sheets were stacked one upon another and heat-treatedin the same manner as that in Example 1 to obtain sintered ferritesubstrates. The obtained substrate was evaluated for its properties andwas found to have a thickness of 171 μm. Since the substrates stuck sotightly to each other it was not possible to peel off the substratesfrom each other. The obtained molded ferrite sheet had surface roughnesssuch that the center line average roughness of 319 nm, the maximumheight of 3.3 μm and the area occupancy rate of the cross-sectionalarea, taken along a horizontal plane at a depth of 50% of the maximumheight in a square of side 100 μm, was 95%. The sintered ferritesubstrate had surface roughness such that the center line averageroughness was 246 nm, the maximum height was 3.3 μm and the areaoccupancy rate of the cross-sectional area, taken along a horizontalplane at a depth of 50% of the maximum height in a square of side 100μm, was 96%.

It will be understood from above results that not only control of thesurface roughness but also control of the area occupancy rate of thecross-sectional area is important in order to obtain the effects of thepresent invention.

TABLE 1 Molded Ferrite Sheet Surface Roughness Bearing Thickness Ra RmaxAnalysis Data (μm) (nm) (μm) Area Rate* (%) Example 1 77 420 6.5 48Example 2 204 370 4.0 73 Example 3 200 270 5.0 20 Example 4 42 435 6.352 Example 5 405 409 6.8 58 Comparative 78 115 1.8 2 Example 1Comparative 202 66 1.3 90 Example 2 Comparative 199 68 1.1 2 Example 3Comparative 49 1120 12.5 23 Example 4 Comparative 200 319 3.3 95 Example5 Sintered Ferrite Substrate Surface Roughness Bearing Thickness Ra RmaxAnalysis Data (μm) (nm) (μm) Area Rate* (%) Example 1 63 400 5.5 45Example 2 170 292 3.5 12 Example 3 167 250 4.0 18 Example 4 37 425 4.950 Example 5 350 388 5.6 41 Comparative 66 98 1.1 2 Example 1Comparative 165 44 0.9 1 Example 2 Comparative 166 51 0.8 2 Example 3Comparative 40 980 9.6 24 Example 4 Comparative 171 246 3.3 96 Example 5Sintered Ferrite Substrate Sticking Magnetic Permeability State μr′ μr″Example 1 No sticking 98 2.2 Example 2 No sticking 96 3 Example 3 Nosticking 80 1.1 Example 4 No sticking 95 2 Example 5 No sticking 102 3.2Comparative Stuck 98 1.9 Example 1 Comparative stuck 97 2.3 Example 2Comparative Partly stuck 96 1.8 Example 3 Comparative No sticking 75 0.6Example 4 Comparative stuck 99 2.2 Example 5 *Area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height

Example 6

A double coated adhesive tape (Product name: 467 MP, manufactured bySumitomo 3M Limited) having a thickness of 50 μm was adhered onto thesurface of the sintered ferrite substrate of Example 1 to whichroughness had been imparted, thereby obtaining a laminate composed of asintered ferrite substrate layer of 63 μm thick and a adhesive layer of50 μm thick. For the purpose of imparting the laminate with bendability,the laminate was placed on an urethane foam sheet having a thickness of10 mm and an expansion ratio of about 10-fold and was then pressed witha rubber roller having an outer dimension of about 50 mm and a width ofabout 15 cm. The rubber roller was displaced in the X and Y directionsof the laminate with a roll linear pressure of about 1 kg/cm so thatcracks were formed throughout the sintered ferrite substrate. A testpiece having an outer diameter of 14 mm and an inner diameter of 8 mmwas cut out and measured for its magnetic permeability. The magneticpermeability was found to have μr′ of 83 and μr″ of 0.8 at 13.56 MHz.Also, the similar laminate was wound around an iron rod having an outerdiameter of 30 mm. In the same manner as above, a test piece was cut outand measured for its magnetic permeability. The magnetic permeabilitywas found to have μr′ of 82.5 and μr″ of 0.8 at 13.56 MHz and wassimilar to the above. Thus, the obtained laminates had good bendabilityand good magnetic permeability μr′ of more than 80.

Example 7

A planar antenna composed of a PET film with a thickness of 25 μm and a7-turn spiral conductive loop provided on the PET film was prepared. Theloop had a rectangular shape with a length of 45 mm and a width of 75mm. A molded ferrite sheet having a thickness of 180 μm was prepared inthe same manner as that in Example 2 using the same Ni—Zn—Cu ferrite asused in Example 1. Using a Thomson blade having V-shaped edges of each30°, V-shaped grooves with a depth of about 90 μm are formed on one sideof the molded sheet. The grooves were arranged at an interval of 2 mm inthe form of a grid. The obtained molded ferrite sheet having grooves wascut into 100 mm squares and the PET film was peeled off. The cut ferritesheets were calcined in the same manner as that in Example 1 to obtainsintered ferrite substrates each having a thickness of 150 μm and anouter dimension of 80 mm square. A conductive paint (Trade Name: DOTITEXE-9000, manufactured by Fujikura Kasei Co., Ltd.) containing silver andcopper powder dispersed in a polyester-based resin was applied to thesurface of the substrate which was not formed with the grooves. Theapplied coating was dried at 50° C. for 30 minutes to form a conductivelayer having a thickness of 30 μm and a surface electric resistance of0.2 Ω/square. To the surface of the conductive layer, a double coatedadhesive tape (Product name: 467 MP, manufactured by Sumitomo 3MLimited) was adhered. To impart bendability to the resulting laminate,the sintered ferrite substrate layer was divided in the same manner asthat in Example 6. The divided pieces were substantially uniform inshape and each in the form of a square of side 2 mm. The magneticpermeability of the laminate sheet was found to have μr′ of 84 and μr″of 0.4.

Then, an antenna module was prepared by bonding the above-obtainedconductive loop antenna and the above-obtained laminate sheet togetherusing a double coated adhesive tape (Product name: 467 MP, manufacturedby Sumitomo 3M Limited) such that the conductive loop of the antennafaces the opposite surface of the sintered ferrite substrate from theconductive layer. The bonding was carried out so that no gaps wereformed in each of the bonding surfaces. Since obtained module had aresonant frequency of the antenna module to 10.8 MHz and Q of 68, acapacitor was connected in parallel to the loop antenna to adjust theresonant frequency to a range of 13.5 to 13.6 by changing the capacity.After the adjustment, the Q value was 64. When the resonant frequencywas measured in a state where the conductive layer of the antenna modulewas in contact with an iron plate having a thickness of 1 mm, no changeof the resonant frequency was observed before and after the attachmentof the iron plate.

Example 8

An antenna module was prepared in the same manner as that in Example 7except that the conductive layer was formed on the sintered ferritesubstrate by applying thereon a nickel-acrylic-based conductive paint(Trade Name: DOTITE FN-101), followed by drying at 50° C. for 30 minutesand had a surface electric resistance of 2 Ω/square. The evaluation ofthe obtained antenna module revealed that the resonant frequency was13.6 MHz and the Q value was 60. No change of the resonant frequency wasobserved before and after the attachment of the iron plate.

Example 9

An antenna module was prepared in the same manner as that in Example 7except that the conductive layer was formed on the sintered ferritesubstrate by printing a conductive silver paste on a green sheet of thesubstrate, followed by sintering the laminate at 900° C. and had athickness of 10 μm. The evaluation of the obtained antenna modulerevealed that the surface electric resistance of the conductive layerwas 0.1 Ω/square, the resonant frequency was 13.55 MHz and the Q valuewas 66. No change of the resonant frequency was observed before andafter the attachment of the iron plate.

Comparative Example 6

An antenna module was prepared in the same manner as that in Example 7except that a conductive layer was not formed on the sintered ferritesubstrate. The obtained antenna module without an iron plate laminatedthereon had a resonant frequency of 13.55 MHz and a Q value of 67. Whenthe resonant frequency was measured with an iron plate of 1 mm thicklaminated in the same manner as that in Example 7, the resonantfrequency was 11.5 MHz and shifted to a low frequency side by 2 MHz,though the Q value was 67 and not changed. Because of the frequencyshift, no resonance occurred at 13.56 MHz. The communication strengthwas considerably low.

Comparative Example 7

An antenna module having the same constitution as that of ComparativeExample 6 except that the thickness of the sintered ferrite substratewas 300 μm was prepared and evaluated. The obtained antenna module had aresonant frequency of 14.1 MHz, when an iron plate was laminatedthereon. Thus, the resonant frequency change was smaller as comparedwith the antenna module of Comparative Example 6. However, thecommunication strength was reduced.

Comparative Example 8

An antenna module having the same constitution as that of Example 7except that the thickness of the conductive layer formed on the sinteredferrite substrate was 5 μm and that the surface electric resistance was5 Ω/square was prepared in the same manner as that in Example 7. Theobtained antenna was measured for its resonance characteristics. It wasfound that the resonant frequency was changed to 10.9 MHz and that thecommunication strength at 13.56 MHz was reduced.

Example 10

Using a pressure kneader, 1,000 parts by weight of a ferrite powder,obtained by surface-treating 1,000 parts by weight of Mg—Zn—Cu ferritepowder (composition: Fe₂O₃: 48.5 mol %, MgO: 27.0 mol %, ZnO: 14.5 mol%, CuO: 10.0 mol %; calcination conditions: 850° C., 180 minutes;cumulative 50% volume diameter: adjusted to 0.7 μm) with 10 parts byweight of a titanate-based coupling agent (KR-TTS manufactured byAjinomoto Co., Inc.), 50 parts by weight of a thermoplastic elastomer(LUMITAC 22-1 manufactured by Tosoh Corporation), 100 parts by weight ofpolyethylene having a density of 0.9 g/cm³ and 20 parts by weight ofstearic acid were kneaded at 130° C. for 40 minutes. The thus obtainedkneaded mass of a ferrite resin composition was press molded at atemperature of 160° C. under a pressure of 100 kg/cm² for apressurization time of 3 minutes using an iron plate, which had beensandblasted to have a center line average roughness (Ra) of 450 nm and amaximum height (Rmax) of 8 μm, to obtain a molded ferrite sheet having athickness of 74 μm and a size of 100 mm square.

Ten such sheets were prepared and stacked one upon another. The stackedsubstrates were placed between top and bottom alumina setters(manufactured by Kikusui Chemical Industries Co., Ltd.) as support platemeans and heated at 500° C. for 10 hours for removing the organicbinder, followed by sintering at 940° C. for 2 hours to obtain sinteredsubstrates. After cooling, the sintered substrates were peeled from eachother. It was found that the sintered substrate was able to be easilypeeled off without causing any damage.

The obtained sintered substrate had a thickness of 60 μm and an outerdimension of 80 mm square. A test piece having an outer diameter of 14mm and an inner diameter of 8 mm was cut out from the substrate andmeasured for its magnetic permeability using Impedance Analyzer (E4991Amanufactured by Agilent Technologies Co., Ltd.) and a jig (16454A)attached to its test station. It was found that μr′ and μr″ were 161 and48, respectively, at 13.56 MHz. Thus, the obtained sintered ferritesubstrates were not stuck to each other and had good magneticcharacteristics.

The above-obtained molded ferrite sheet was found to have a surfaceroughness such that the center line average roughness (Ra) was 380 nm,the maximum height (Rmax) was 4.8 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 38%.

The obtained sintered ferrite substrate was found to have a surfaceroughness such that the center line average roughness (Ra) was 366 nm,the maximum height (Rmax) was 4.1 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 31%.

Example 11

Using a ball mill, 100 parts by weight of the same Mg—Zn—Cu ferrite asused in Example 10, 2 parts by weight of butyl phthalyl butyl glycolate,12 parts of a polyvinylalcohol resin (ESLEK B BM-1 manufactured bySekisui Chemical Co., Ltd.) and 60 parts by weight of a mixed solventcomposed of 4 parts of n-butanol and 6 parts of toluene were mixed,dissolved or dispersed to obtain a ferrite-dispersed coating liquid. Thetermite-dispersed coating liquid was defoamed by an oil rotary vacuumpump and uniformly applied with a doctor blade to a PET film (LUMIMAT50S200 TRES manufactured by Panak Co., Ltd.), one side of which had beensandblasted to have a center line average roughness (Ra) of 530 nm and amaximum height (Rmax) of 5.6 μm, to a given thickness. The coating wasdried with hot wind at 100° C. for 30 minutes to obtain a molded ferritesheet having a thickness of 210 μm.

The molded ferrite sheet was cut into squares of side 100 mm. Each ofthe cut sheets was peeled off from the PET film. The sheets were thensintered in the same conditions as those in Example 10 to obtainsintered ferrite substrates.

The obtained sintered ferrite substrate was evaluated for its physicalproperties and was found to have a thickness of 174 μm, an outerdimension of 80 mm square and magnetic permeability with μr′ of 158 andμr″ of 33. Further, the obtained sintered ferrite substrates did notstick to each other and were easily peeled off from each other.

The above-obtained molded ferrite sheet was found to have a surfaceroughness such that the center line average roughness (Ra) was 450 nm,the maximum height (Rmax) was 5.1 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 40%.

The surface of the above-obtained molded ferrite sheet which had notbeen brought into contact with the PET film was found to have a centerline average roughness (Ra) of 131 nm, a maximum height (Rmax) of 1.8 μmand an area occupancy rate of the cross-sectional area, taken along ahorizontal plane at a depth of 50% of the maximum height in a square ofside 100 μm, of 97%. Thus, it was understood that the surface roughnesswas able to be controlled at will by the PET film used.

The above-obtained molded ferrite sheet was found to have a surfaceroughness such that the center line average roughness (Ra) was 338 nm,the maximum height (Rmax) was 3.6 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, was 21%.

Example 12

A kneaded mass of a ferrite resin composition was prepared in the samemanner as that in Example 10 except that a mixture of 300 parts byweight of Mg—Zn—Cu ferrite powder (composition: Fe₂O₃: 48.5 mol %, MgO:27.0 mol %, ZnO: 14.5 mol %, CuO: 10.0 mol %; calcination conditions:1,000° C., 180 minutes; cumulative 50% volume diameter: 6 μm) and 700parts by weight of the same Mg—Zn—Cu ferrite powder (cumulative 50%volume diameter: 0.7 μm) as used in Example 10 was used. The thusobtained kneaded mass was press molded at a temperature of 160° C. undera pressure of 100 kg/cm² for a pressurization time of 3 minutes using aniron plate, which had been processed to have a center line averageroughness (Ra) of 120 nm and a maximum roughness of 2 μm, to obtain amolded ferrite sheet having a thickness of 188 μm and an outer size of100 mm.

Using the thus obtained sheet, sintered ferrite substrates were preparedin the same manner as that in Example 10. The obtained sintered ferritesubstrate was evaluated for its physical properties and was found tohave a thickness of 157 μm and magnetic permeability with μr′ of 144 andμr″ of 21 at 13.56 MHz. Further, the substrates did not stick to eachother and were easily peeled off from each other.

Because of the use of coarse particle ferrite, the above-obtained moldedferrite sheet had a center line average roughness (Ra) of 361 nm, amaximum height (Rmax) of 6.2 μm and an area occupancy rate of thecross-sectional area, taken along a horizontal plane at a depth of 50%of the maximum height in a square of side 100 μm, of 67%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 305 nm, the maximum height (Rmax)was 4.0 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 49%.

Example 13

Sintered ferrite substrates were obtained in the same manner as that inExample 11 except that the application of the ferrite-dispersed coatingliquid using the doctor blade was carried out under conditions so that amolded ferrite sheet obtained had a thickness of 43 μm.

The obtained sintered ferrite substrate was evaluated for its physicalproperties and was found to have a thickness of 37 μm and magneticpermeability with μr′ of 156 and μr″ of 31 at 13.56 MHz. Further, thesubstrates did not stick to each other and were easily peeled off fromeach other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness (Ra) of 345 nm, the maximum height (Rmax)of 4.0 μm and the area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 23%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 289 nm, the maximum height (Rmax)was 3.1 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 12%.

Example 14

Sintered ferrite substrates were obtained in the same manner as that inExample 11 except that the application of the ferrite-dispersed coatingliquid using the doctor blade was carried out under conditions so that amolded ferrite sheet obtained had a thickness of 377 μm.

The obtained sintered ferrite substrate was evaluated for its physicalproperties and was found to have a thickness of 326 μm and magneticpermeability with μr′ of 167 and μr″ of 50 at 13.56 MHz. Further, thesubstrates did not stick to each other and was easily peeled off fromeach other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness (Ra) of 634 nm, the maximum height (Rmax)of 7.8 μm and the area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 66%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 593 nm, the maximum height (Rmax)was 7.8 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 39%.

Comparative Example 9

A kneaded mass of a ferrite resin composition was prepared in the samemanner as that in Example 10. Using the kneaded mass, sintered ferritesubstrates were prepared in the same manner as that in Example 10 exceptthat an iron plate processed to have a center line average roughness(Ra) of 120 nm and a maximum roughness of 2 μm was used in the pressmolding. The substrates stuck so tightly to each other that it wasdifficult to peel off the substrates from each other. Although thesubstrate was partly unstuck from each other, breakage occurred when itwas forcibly separated therefrom. Thus, no sintered ferrite substrateshaving a size of 80 mm square were obtained. The obtained sinteredferrite substrate had magnetic permeability with μr′ of 160 and μr″ of48 at 13.56 MHz.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness (Ra) of 98 nm, the maximum height (Rmax)of 0.9 μm and the area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 5%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 81 nm, the maximum height (Rmax)was 0.8 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 1%.

Comparative Example 10

A ferrite-dispersed coating liquid was prepared in the same manner asthat in Example 11. The obtained coating liquid was uniformly appliedwith a doctor blade to a PET film (not sandblasted) having a center lineaverage roughness (Ra) of 17 nm, a maximum height (Rmax) of 0.3 μm and athickness of 50 μm. This was dried with hot wind at 100° C. for 30minutes to obtain a molded ferrite sheet having a thickness of 217 μm.

The sheet was peeled off from the PET film. Ten such sheets were stackedone upon another and heated in the same manner as that in Example 10 toobtain sintered ferrite substrates. The obtained substrate was evaluatedfor its properties and was found to have a thickness of 177 μm. Sincethe substrates stuck so tightly to each other it was not possible topeel off the substrates from each other.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness (Ra) of 78 nm, the maximum height (Rmax)of 1.8 μm and the area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 87%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 54 nm, the maximum height (Rmax)was 1.3 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 0.2%.

Comparative Example 11

A sheet was prepared in the same manner as that in Comparative Example10 and was peeled off from the PET film. That surface of the moldedferrite sheet which had been in contact with the PET film was appliedwith 50 mg of zirconia powder having an average particle diameter of 5μm by brushing. Thereafter, a calcination treatment was carried out inthe same manner as that in Comparative Example 10 to obtain sinteredferrite substrates.

The evaluation of the sintered ferrite substrate revealed that thesubstrate had magnetic permeability with μr′ of 157 and μr″ of 31 at13.56 MHz. The obtained sintered ferrite substrates had deposits ofzirconia powder. During the removal of the zirconia powder depositsusing a brush, four of the ten substrates were broken. The applicationof the powder and the removal of the powder were significantlytroublesome. The powder deposits were not able to be completely removed.

Comparative Example 12

Sintered ferrite substrates were prepared in the same manner as that inExample 10 except that an iron plate processed to have a center lineaverage roughness (Ra) of 1200 nm and a maximum height (Rmax) of 14 μmwas used in the press molding. The substrates did not stuck to eachother and were able to be peeled off from each other.

The obtained sintered ferrite substrate had magnetic permeability withμr′ of 78 and μr″ of 1 at 13.56 MHz and, thus, did not have satisfactorymagnetic characteristics. Such deterioration of the magneticpermeability is considered to be attributed to the high surfaceroughness of the iron plate which resulted in an increase of void spacesin the cross-sections of the sintered ferrite substrate.

Comparative Example 13

A ferrite-dispersed coating liquid was prepared in the same manner asthat in Example 11. The obtained coating liquid was uniformly appliedwith a doctor blade to a PET film (U4-50 manufactured by Teijin DuPontCo., Ltd) which had been processed to have a center line averageroughness (Ra) of 252 nm and a maximum height (Rmax) of 3.3 μm. Thecoating was dried with hot wind at 100° C. for 30 minutes to obtain amolded ferrite sheet having a thickness of 198 μm. The sheet was peeledoff from the PET film. Ten such sheets were stacked one upon another andheat-treated in the same manner as that in Example 10 to obtain sinteredferrite substrates.

The obtained substrate was evaluated for its properties and was found tohave a thickness of 169 μm. Since the substrates stuck so tightly toeach other it was not possible to peel off the substrates from eachother.

The obtained molded ferrite sheet had surface roughness such that thecenter line average roughness (Ra) of 246 nm, the maximum height (Rmax)of 2.6 μm and the area occupancy rate of the cross-sectional area, takenalong a horizontal plane at a depth of 50% of the maximum height in asquare of side 100 μm, was 97%.

The sintered ferrite substrate had surface roughness such that thecenter line average roughness (Ra) was 201 nm, the maximum height (Rmax)was 2.1 μm and the area occupancy rate of the cross-sectional area,taken along a horizontal plane at a depth of 50% of the maximum heightin a square of side 100 μm, was 96%.

It will be understood from above results that not only control of thesurface roughness but also control of the area occupancy rate of thecross-sectional area is important in order to obtain the effects of thepresent invention.

TABLE 2 Molded Ferrite Sheet Surface Roughness Bearing Thickness Ra RmaxAnalysis Data (μm) (nm) (μm) Area Rate* (%) Example 10 74 380 4.8 38Example 11 210 450 5.1 40 Example 12 188 361 6.2 67 Example 13 43 3454.0 23 Example 14 377 634 7.8 66 Comparative 82 98 0.9 5 Example 9Comparative 217 78 1.8 87 Example 10 Comparative 180 60 1.2 5 Example 11Comparative 53 1005 14.2 65 Example 12 Comparative 198 246 2.6 97Example 13 Sintered Ferrite Substrate Sintered Surface RoughnessThickness Density Ra Rmax (μm) (g/cm³) (nm) (μm) Example 10 60 4.64 3664.1 Example 11 174 4.61 338 3.6 Example 12 157 4.59 305 4.0 Example 1337 4.62 289 3.1 Example 14 326 4.71 593 7.8 Comparative 70 4.63 81 0.8Example 9 Comparative 177 4.62 54 1.3 Example 10 Comparative 149 4.64 431.1 Example 11 Comparative 43 4.38 922 10.9 Example 12 Comparative 1694.67 201 2.1 Example 13 Sintered Ferrite Substrate Bearing Analysis DataSticking Magnetic Permeability Area Rate* (%) State μr′ μr″ Example 1031 No sticking 161 48 Example 11 21 No sticking 158 33 Example 12 49 Nosticking 144 21 Example 13 12 No sticking 156 31 Example 14 39 Nosticking 167 50 Comparative 1 Stuck 160 48 Example 9 Comparative 0.2stuck 159 44 Example 10 Comparative 2 Partly stuck 157 31 Example 11Comparative 55 No sticking 78 1.1 Example 12 Comparative 96 stuck 165 50Example 13 *Area occupancy rate of the cross-sectional area, taken alonga horizontal plane at a depth of 50% of the maximum height

Example 15

A double coated adhesive tape (Product name: 467 MP, manufactured bySumitomo 3M Limited) having a thickness of 50 μm was adhered onto thesurface of the sintered ferrite substrate of Example 10 to whichroughness had been imparted, thereby obtaining a laminate composed of asintered ferrite substrate layer of 60 μm thick and a adhesive layer of50 μm thick.

For the purpose of imparting the laminate with bendability, the laminatewas placed on an urethane foam sheet having a thickness of 10 mm and anexpansion ratio of about 10-fold and was then pressed with a rubberroller having an outer dimension of about 50 mm and a width of about 15cm. The rubber roller was displaced in the X and Y directions of thelaminate with a roll linear pressure of about 1 kg/cm so that crackswere formed throughout the sintered ferrite substrate.

A test piece having an outer diameter of 14 mm and an inner diameter of8 mm was cut out and measured for its magnetic permeability. Themagnetic permeability was found to have μr′ of 121 and μr″ of 10 at13.56 MHz.

Also, the similar laminate was wound around an iron rod having an outerdiameter of 30 mm. In the same manner as above, a test piece was cut outand measured for its magnetic permeability. The magnetic permeabilitywas found to have μr′ of 120 and μr″ of 10 at 13.56 MHz and was similarto the above. Thus, the obtained laminates had good bendability and goodmagnetic permeability μr′ of more than

Example 16

A planar antenna composed of a PET film with a thickness of 25 μm and a7-turn spiral conductive loop provided on the PET film was prepared. Theloop had a rectangular shape with a length of 45 mm and a width of 75mm.

A molded ferrite sheet having a thickness of 185 μm was prepared in thesame manner as that in Example 11 using the same Mg—Zn—Cu ferrite asused in Example 10. Using a Thomson blade having V-shaped edges of each30°, V-shaped grooves with a depth of about 90 μm are formed on one sideof the molded sheet. The grooves were arranged at an interval of 2 mm inthe form of a grid. The obtained molded ferrite sheet having grooves wascut into 100 mm squares and the PET film was peeled off. The cut ferritesheets were calcined in the same manner as that in Example 10 to obtainsintered ferrite substrates.

The obtained sintered ferrite substrate had a thickness of 143 μm and anouter dimension of 80 mm square. A conductive paint (Trade Name: DOTITEXE-9000, manufactured by Fujikura Kasei Co., Ltd.) containing silver andcopper powder dispersed in a polyester-based resin was applied to thesurface of the substrate which was not formed with the grooves. Theapplied coating was dried at 50° C. for 30 minutes to form a conductivelayer having a thickness of 30 μm and a surface electric resistance of0.2 Ω/square.

To the surface of the conductive layer, a double coated adhesive tape(Product name: 467 MP, manufactured by Sumitomo 3M Limited) was adhered.To impart bendability to the resulting laminate, the sintered ferritesubstrate layer was divided in the same manner as that in Example 15.The divided pieces were substantially uniform in shape and each in theform of a square of side 2 mm. The magnetic permeability of the laminatesheet was found to have μr′ of 119 and μr″ of 9.0.

Then, an antenna module was prepared by bonding the above-obtainedconductive loop antenna and the above-obtained laminate sheet togetherusing a double coated adhesive tape (Product name: 467 MP, manufacturedby Sumitomo 3M Limited) such that the conductive loop of the antennafaces the opposite surface of the sintered ferrite substrate from theconductive layer. The bonding was carried out so that no gaps wereformed in each of the bonding surfaces. Since obtained module had aresonant frequency of the antenna module to 15.5 MHz and Q of 67, acapacitor was connected in parallel to the loop antenna to adjust theresonant frequency to a range of 13.5 to 13.6 by changing the capacity.No change of Q was seen after the adjustment. When the resonantfrequency was measured in a state where the conductive layer of theantenna module was in contact with an iron plate having a thickness of 1mm, no change of the resonant frequency was observed before and afterthe attachment of the iron plate.

Example 17

An antenna module was prepared in the same manner as that in Example 16except that the conductive layer was formed on the sintered ferritesubstrate by applying thereon a nickel-acrylic-based conductive paint(Trade Name: DOTITE FN-101), followed by drying at 50° C. for 30 minutesand had a surface electric resistance of 2 Ω/square. The evaluation ofthe obtained antenna module revealed that the resonant frequency was13.6 MHz and the Q value was 63. No change of the resonant frequency wasobserved before and after the attachment of the iron plate.

Example 18

An antenna module was prepared in the same manner as that in Example 16except that the conductive layer was formed on the sintered ferritesubstrate by printing a conductive silver paste on a green sheet of thesubstrate, followed by sintering the laminate at 900° C. and had athickness of 10 μm. The evaluation of the obtained antenna modulerevealed that the surface electric resistance of the conductive layerwas 0.1 Ω/square, the resonant frequency was 13.55 MHz and the Q valuewas 63. No change of the resonant frequency was observed before andafter the attachment of the iron plate.

Comparative Example 14

An antenna module was prepared in the same manner as that in Example 16except that a conductive layer was not formed on the sintered ferritesubstrate. The obtained antenna module without an iron plate laminatedthereon had a resonant frequency of 13.5 MHz and a Q value of 66. Whenthe resonant frequency was measured with an iron plate of 1 mm thicklaminated in the same manner as that in Example 13, the resonantfrequency was 15.8 MHz and shifted to a high frequency side by 2.3 MHz,though the Q value was 66 and not changed. Because of the frequencyshift, no resonance occurred at 13.56 MHz. The communication strengthwas considerably low.

Comparative Example 15

An antenna module having the same constitution as that of ComparativeExample 14 except that the thickness of the sintered ferrite substratewas 300 μm was prepared and evaluated. The obtained antenna module had aresonant frequency of 13.9 MHz, when an iron plate was laminatedthereon. Thus, the resonant frequency change was smaller as comparedwith the antenna module of Comparative Example 14. However, thecommunication strength was reduced.

Comparative Example 16

An antenna module having the same constitution as that of Example 7except that the thickness of the conductive layer formed on the sinteredferrite substrate was 5 μm and that the surface electric resistance was5 Ω/square was prepared in the same manner as that in Example 16. Theobtained antenna was measured for its resonance characteristics. It wasfound that the resonant frequency was changed to 15.0 MHz and that thecommunication strength at 13.56 MHz was reduced.

1. A molded ferrite sheet having opposing surfaces and a thickness in arange of 30 μm to 430 μm, at least one surface of said opposing surfaceshaving the following surface roughness characteristics (a) to (c): (a) acenter line average roughness is in a range of 170 nm to 800 nm, (b) amaximum height is in a range of 3 μm to 10 μm, and (c) an area occupancyrate of cross-sectional area taken along a horizontal plane at a depthof 50% of the maximum height in a square of side 100 μm is in a range of10 to 80%.
 2. A molded ferrite sheet as recited in claim 1, wherein saidat least one surface is roughened by sandblasting.
 3. A molded ferritesheet as recited in claim 1, wherein said molded ferrite sheet isprepared by molding under pressure using a mold or calender roll havinga roughened surface so that the roughness of the roughened surface ofsaid mold or calender roll is transferred to a surface of said moldedferrite sheet in contact with the roughened surface of said mold orcalender roll.
 4. A molded ferrite sheet as recited in claim 1, whereinsaid molded ferrite sheet is prepared by a method which comprisesapplying a coating of a ferrite-dispersed coating liquid to a surface ofa plastic film, and drying the applied coating, and wherein said surfaceof said plastic film has been roughened by sandblasting so that theroughness of said plastic film is transferred to a surface of the driedcoating in contact with the roughened surface of said plastic film.
 5. Amolded ferrite sheet as recited in claim 1, wherein said molded ferritesheet is prepared by a method which comprises applying a coating of aferrite-dispersed coating liquid to a support and drying the appliedcoating, and wherein the ferrite has been obtained by adjusting aparticle size of a ferrite powder having an average particle diameter of0.1 to 10 μm so that said surface roughness characteristics (a) to (c)are imparted to a surface of the dried coating in contact with saidsupport.
 6. A molded ferrite sheet as recited in claim 1, wherein theferrite is Ni—Zn—Cu-based spinel ferrite or Mg—Zn—Cu-based spinelferrite. 7.-18. (canceled)